Why would you want accurate quasar redshifts?

Paul Hewett (IoA, Cambridge)

James Allen (U. Sydney)

Outline• Motivation: quasar clustering, host-galaxy and local

quasar environment, relation to Lyα-absorbers and the inter-galactic medium, outflow properties of quasars themselves

• Current redshift accuracy for high-redshift, z>2.0, quasars and what do we need?

• Decomposing spectra into components to do better - redshifts using Independent Component Analysis applied to the ~1900A CIII]+SiIII]+AlIII complex

• Comparison to BOSS DR10 pipeline and PCA redshifts• What is now possible

Galaxies2dF Galaxy Survey redshift wedge

Redshift due to `Hubble flow’component plus`peculiar velocity’ due to influence of mass inhomogeneities over age of Universe

For optical galaxy spectra can reach σ~30km/s

For Hubble Constant ~70km/s/Mpcapprox 0.5Mpc λObserved = λRestframe×(1+z) with z=redshift

Want same for quasars but also have ability to study relation of quasars to gas along line-of-sight: probes host-galaxy, local environment and inter-galactic medium

(Michael Murphy)

Last decade produced revolution in quasar surveys – Sloan Digital Sky Survey (SDSS)- DR7 100000 quasars (2007); DR12 (BOSS) 450000 quasars (2014)- 3D quasar clustering and 3D quasar-absorber studies possible- Quasar host-galaxy and environment studies also viable

Wild+2008

Blackhole, fed by accretion disk with associated Broad Line Region (BLR) clouds and more distant Narrow Line Region (NLR) cloudsCloud distances ~1 parsec (BLR), 1000 parsec (NLR) with associated velocities ~5000 and 1000km/s respectively

NASA

• Rest-frame ultraviolet and optical quasar spectrum • Continuum (from accretion disk) broad and narrow emission lines• Encouraging in that emission from common elements evident• At low-redshift, see host-galaxy spectrum and quasar• Can work up to higher redshift using emission lines with rest-frame

wavelengths >2500Ǻ - find redshift accuracy no worse than σ~170km/s• BUT at shorter wavelengths, equivalent to redshift z>2.0 for ground-based

spectra, quasar spectral energy distribution (SED) shows significant variations

• Observationally, strong asymmetries (blueshifts) evident for high-ionization emission lines (Gaskell 1982, Carswell, Tytler,…), i.e the broad emission lines have different shapes

• One explanation invokes presence of `disk winds’ with material at high outflow velocities contributing to the emission-line profiles

• Systematic dependence on viewing orientation, L/LEddington , luminosity relative to Eddington luminosity,…

• Not a subtle effect – 3000km/s shifts mean redshifts awry by up to 100× galaxy redshift errors [~15Mpc Hubble flow]

• True for optical spectra of z>2 quasars – key epoch and essential for quasar-IGM studies

• Relationship between quasar and environment also severely compromised

Observed frequency distribution of redshift differences, , for 23 800 C iv absorbers using both SDSS (blue) and HW (red) redshifts for quasars with redshifts 1.55< z <3.5.

Paul C. Hewett, and Vivienne Wild MNRAS 2010;405:2302-2316

© 2010 The Authors. Journal compilation © 2010 RAS

• Low ionization lines, including MgII 2800 provide stable reference – good for redshifts z<2.0 but z>2.0 key

• Hewett+Wild (2010) scheme major improvement for SDSS DR7 but essentially based on a single-”template”

• All single-template schemes have no information on spectra differences – hence SED-dependent systematic errors

• Amplitude of systematics >1000km/s, whereas want ~200km/s for host-galaxy, environment, clustering,…investigations

• Widely recognised that a natural solution involves– Parametrize quasar spectra into a number of “components”– Each quasar represented by sum of components with different weights – Reconstruct spectrum of each quasar determining component weights

and redshift similtaneously– Allows for SED-variation, e.g. component(s) might include `outflow’

signature and be present in different amounts from quasar to quasar

Quasar Redshifts: Status

• Spectra as a linear combination of components S = W C

spectra = weights × components

Good if C<<S• Differing rules/constraints on “component” derivation

– Principal Component Analysis (PCA)– Independent Component Analysis (ICA)

• Mean Field ICA (Allen, Hewett+ 2013 MN 430 3510)– Very different from most ICA implementations– Priors, constraints on components possible– Extremely compact (i.e. #components small)– Example using SDSS low-z Post starburst galaxies (“answer

known”)

Decomposing Spectra

• For the SDSS BOSS quasar survey [450000 quasars nearly all with z>2]

• Two main redshift estimates – Z_VI (pipeline/visual inspection) – Z_PCA (Paris+ 2011,2012) hope to be SED-independent

• Z_PCA unbiased, σ~750km/s relative to MgII 2800 – not great• Statistical analysis using Lyα-forest

– Cross-correlation with Lyα-forest gives Z_VI=231km/s low, Z_PCA=154km/s low (Font-Ribera 2013) [both +/-30km/s]

• Both BOSS schemes use quasar spectra down to 1400Ǻ• MFICA components [6] applied to just the low ionization CIII]

+SiIII]+AlIII complex. ~2400 BOSS quasars, @z~2.5 where MgII visible [use well-behaved MgIIλ2800 as reference]

SDSS DR12 Quasar Redshifts

Current Status

• Confirmation of BOSS-projects own redshift uncertainty determinations

• MFICA CIII]-complex redshifts reduce errors to σ~200km/s (cf. current σ~750km/s)– Possible for SDSS DR7 and BOSS to z<4.0– Possible for any quasar spectrum where CIII]-

complex covered• Accurate individual errors from an MCMC scheme

using component weight and redshift errors – on the way

• Quasar environments (host-galaxy, group/clusters,…) and absorber outflow properties as a function of black-hole mass, L/LEddington, radio-properties,…

• Example from Wild (2009) for MgII 2800 absorbers – host(?), outflow and intervening absorber components defined

• Full SDSS DR7 and DR12 analysis will produce vast improvement in statistics

Summary

• Many astrophysical investigations involving quasars at z>2 limited by errors in redshift determinations

• MFICA CIII]-complex redshifts reduce errors to σ~200km/s (cf. current σ~750km/s)– Possible for SDSS DR7 and BOSS to z<4.5– Possible for any quasar spectrum where CIII]-

complex covered